memcg: fix deadlock between cpuset and memcg

Commit b1dd693e ("memcg: avoid deadlock between move charge and
try_charge()") can cause another deadlock about mmap_sem on task migration
if cpuset and memcg are mounted onto the same mount point.

After the commit, cgroup_attach_task() has sequence like:

cgroup_attach_task()
ss->can_attach()
cpuset_can_attach()
mem_cgroup_can_attach()
down_read(&mmap_sem) (1)
ss->attach()
cpuset_attach()
mpol_rebind_mm()
down_write(&mmap_sem) (2)
up_write(&mmap_sem)
cpuset_migrate_mm()
do_migrate_pages()
down_read(&mmap_sem)
up_read(&mmap_sem)
mem_cgroup_move_task()
mem_cgroup_clear_mc()
up_read(&mmap_sem)

We can cause deadlock at (2) because we've already aquire the mmap_sem at (1).

But the commit itself is necessary to fix deadlocks which have existed
before the commit like:

Ex.1)
move charge | try charge
--------------------------------------+------------------------------
mem_cgroup_can_attach() | down_write(&mmap_sem)
mc.moving_task = current | ..
mem_cgroup_precharge_mc() | __mem_cgroup_try_charge()
mem_cgroup_count_precharge() | prepare_to_wait()
down_read(&mmap_sem) | if (mc.moving_task)
-> cannot aquire the lock | -> true
| schedule()
| -> move charge should wake it up

Ex.2)
move charge | try charge
--------------------------------------+------------------------------
mem_cgroup_can_attach() |
mc.moving_task = current |
mem_cgroup_precharge_mc() |
mem_cgroup_count_precharge() |
down_read(&mmap_sem) |
.. |
up_read(&mmap_sem) |
| down_write(&mmap_sem)
mem_cgroup_move_task() | ..
mem_cgroup_move_charge() | __mem_cgroup_try_charge()
down_read(&mmap_sem) | prepare_to_wait()
-> cannot aquire the lock | if (mc.moving_task)
| -> true
| schedule()
| -> move charge should wake it up

This patch fixes all of these problems by:
1. revert the commit.
2. To fix the Ex.1, we set mc.moving_task after mem_cgroup_count_precharge()
has released the mmap_sem.
3. To fix the Ex.2, we use down_read_trylock() instead of down_read() in
mem_cgroup_move_charge() and, if it has failed to aquire the lock, cancel
all extra charges, wake up all waiters, and retry trylock.

Signed-off-by: Daisuke Nishimura <[e-mail]>
Reported-by: Ben Blum <[e-mail]>
Cc: Miao Xie <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: Paul Menage <[e-mail]>
Cc: Hiroyuki Kamezawa <[e-mail]>
Cc: Balbir Singh <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memcontrol.c

···
 	unsigned long moved_charge;
 	unsigned long moved_swap;
 	struct task_struct *moving_task;	/* a task moving charges */
-	struct mm_struct *mm;
 	wait_queue_head_t waitq;		/* a waitq for other context */
 } mc = {
 	.lock = __SPIN_LOCK_UNLOCKED(mc.lock),
···
 	unsigned long precharge;
 	struct vm_area_struct *vma;
 
-	/* We've already held the mmap_sem */
+	down_read(&mm->mmap_sem);
 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
 		struct mm_walk mem_cgroup_count_precharge_walk = {
 			.pmd_entry = mem_cgroup_count_precharge_pte_range,
···
 		walk_page_range(vma->vm_start, vma->vm_end,
 					&mem_cgroup_count_precharge_walk);
 	}
+	up_read(&mm->mmap_sem);
 
 	precharge = mc.precharge;
 	mc.precharge = 0;
···
 
 static int mem_cgroup_precharge_mc(struct mm_struct *mm)
 {
-	return mem_cgroup_do_precharge(mem_cgroup_count_precharge(mm));
+	unsigned long precharge = mem_cgroup_count_precharge(mm);
+
+	VM_BUG_ON(mc.moving_task);
+	mc.moving_task = current;
+	return mem_cgroup_do_precharge(precharge);
 }
 
-static void mem_cgroup_clear_mc(void)
+/* cancels all extra charges on mc.from and mc.to, and wakes up all waiters. */
+static void __mem_cgroup_clear_mc(void)
 {
 	struct mem_cgroup *from = mc.from;
 	struct mem_cgroup *to = mc.to;
···
 						PAGE_SIZE * mc.moved_swap);
 		}
 		/* we've already done mem_cgroup_get(mc.to) */
-
 		mc.moved_swap = 0;
 	}
-	if (mc.mm) {
-		up_read(&mc.mm->mmap_sem);
-		mmput(mc.mm);
-	}
+	memcg_oom_recover(from);
+	memcg_oom_recover(to);
+	wake_up_all(&mc.waitq);
+}
+
+static void mem_cgroup_clear_mc(void)
+{
+	struct mem_cgroup *from = mc.from;
+
+	/*
+	 * we must clear moving_task before waking up waiters at the end of
+	 * task migration.
+	 */
+	mc.moving_task = NULL;
+	__mem_cgroup_clear_mc();
 	spin_lock(&mc.lock);
 	mc.from = NULL;
 	mc.to = NULL;
 	spin_unlock(&mc.lock);
-	mc.moving_task = NULL;
-	mc.mm = NULL;
 	mem_cgroup_end_move(from);
-	memcg_oom_recover(from);
-	memcg_oom_recover(to);
-	wake_up_all(&mc.waitq);
 }
 
 static int mem_cgroup_can_attach(struct cgroup_subsys *ss,
···
 			return 0;
 		/* We move charges only when we move a owner of the mm */
 		if (mm->owner == p) {
-			/*
-			 * We do all the move charge works under one mmap_sem to
-			 * avoid deadlock with down_write(&mmap_sem)
-			 * -> try_charge() -> if (mc.moving_task) -> sleep.
-			 */
-			down_read(&mm->mmap_sem);
-
 			VM_BUG_ON(mc.from);
 			VM_BUG_ON(mc.to);
 			VM_BUG_ON(mc.precharge);
 			VM_BUG_ON(mc.moved_charge);
 			VM_BUG_ON(mc.moved_swap);
-			VM_BUG_ON(mc.moving_task);
-			VM_BUG_ON(mc.mm);
-
 			mem_cgroup_start_move(from);
 			spin_lock(&mc.lock);
 			mc.from = from;
 			mc.to = mem;
-			mc.precharge = 0;
-			mc.moved_charge = 0;
-			mc.moved_swap = 0;
 			spin_unlock(&mc.lock);
-			mc.moving_task = current;
-			mc.mm = mm;
+			/* We set mc.moving_task later */
 
 			ret = mem_cgroup_precharge_mc(mm);
 			if (ret)
 				mem_cgroup_clear_mc();
-			/* We call up_read() and mmput() in clear_mc(). */
-		} else
-			mmput(mm);
+		}
+		mmput(mm);
 	}
 	return ret;
 }
···
 	struct vm_area_struct *vma;
 
 	lru_add_drain_all();
-	/* We've already held the mmap_sem */
+retry:
+	if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
+		/*
+		 * Someone who are holding the mmap_sem might be waiting in
+		 * waitq. So we cancel all extra charges, wake up all waiters,
+		 * and retry. Because we cancel precharges, we might not be able
+		 * to move enough charges, but moving charge is a best-effort
+		 * feature anyway, so it wouldn't be a big problem.
+		 */
+		__mem_cgroup_clear_mc();
+		cond_resched();
+		goto retry;
+	}
 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
 		int ret;
 		struct mm_walk mem_cgroup_move_charge_walk = {
···
 			 */
 			break;
 	}
+	up_read(&mm->mmap_sem);
 }
 
 static void mem_cgroup_move_task(struct cgroup_subsys *ss,
···
 				struct task_struct *p,
 				bool threadgroup)
 {
-	if (!mc.mm)
+	struct mm_struct *mm;
+
+	if (!mc.to)
 		/* no need to move charge */
 		return;
 
-	mem_cgroup_move_charge(mc.mm);
+	mm = get_task_mm(p);
+	if (mm) {
+		mem_cgroup_move_charge(mm);
+		mmput(mm);
+	}
 	mem_cgroup_clear_mc();
 }
 #else	/* !CONFIG_MMU */